In Situ Assembly of Hydrogen‐Bonded Organic Framework on Metal–Organic Framework: An Effective Strategy for Constructing Core–Shell Hybrid Photocatalyst

Abstract The hydrogen‐bonded organic frameworks (HOFs) have rarely been considered for photocatalytic application, given their weak stability and low activity. One presumably effective strategy to improve the photocatalytic performance of the HOFs is to produce a core–shell composite by fabricating a particular nanostructure using stable HOFs. To this end, the surface‐functionalized metal–organic frameworks (MOFs) are used as the host matrix to support the in situ assembly and subsequent multisite growth of the stable HOFs. MOF@HOF eventually obtains core–shell hybrids, i.e., NH2‐UiO‐66@DAT‐HOF. This newly synthesized core–shell nanostructure exhibits excellent stability and superb photocatalytic performance. For example, in terms of tetracycline degradation, the optimal composite presents an apparent reaction rate constant of 60.7 and 7.6 times higher than its parent materials NH2‐UiO‐66 and DAT‐HOF. Such a pronounced enhancement in photocatalytic efficiency of the hybrid material is attributed to the broader visible‐light utilization range compared to its individual parent material as well as the efficient separation of charge carriers supported by the S‐scheme heterojunction. In addition, it is particularly notable that the photocatalytic efficiency of the yielded core–shell nanostructure can remain high after several‐cycle applications. This work provides a universal scheme for synthesizing the MOF@HOF core–shell hybrids.


Preparation of NH 2 -MIL-68 (MIL
were dissolved in 10 mL of HAc. Then, the mixture was stirred vigorously and heated at 119 °C for 5 hours. Upon cooling, the resulted solid was washed with DMF and methanol, then lyophilized for 24 hours to obtain anhydride functionalized MIL. In step two, DAT (99.1 mg, 1.00 mmol) was dissolved in 10 mL of DMAc in an ice bath under N 2 for 30 min, then NTCDA (134.1 mg, 0.50 mmol) was added and the mixture was stirred vigorously in an ice bath for another 30 min. Subsequently, the obtained anhydride functionalized MIL and 60 mL of DMAc were added into the mixture, followed by stirring in an ice bath for 5 min. The above was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 170 °C for 6 hours. After cooling to room temperature, the resulted solid was washed with DMF and methanol, then lyophilized for 24 hours to obtain MIL@H.
All the yields of samples were given and presented in Table S1.

Photocatalytic experiments:
The photocatalytic degradation experiments were carried out under Xe lamp irradiation with a cut-off filter (300 W, Light intensity = 100 mW cm -2 , λ > 400 nm). 20 mg photocatalyst was added to 100 mL of TC solution (50 mg L -1 ) and the suspension was stirred in the dark for 60 min before irradiation. Then the Xe lamp was turned on and 1.0 mL of the suspension was taken out per 10 minutes.

Metal photodeposition experiments:
Photodeposition of Au and PbO 2 on the surfaces of U@H2 were carried out using H 2 AuCl 4 and Pb(NO 3 ) 2 as precursors, respectively.
Typically, 20 mg of U@H2 and the metal precursor were mixed in 100 mL of solvent with stirring. In the photo deposition of Au, 2 mg of H 2 AuCl 4 was added and the mixture of distilled water (80 mL) and isopropanol (20 mL), then the precursor was degassed and bubbled with N 2 . The suspension was then irradiated by a 300 W Xe lamp. After 4 h of photo deposition, the suspension was filtered, washed several times with deionized water, and finally dried in the oven at 60 °C overnight. In the photo deposition of PbO2, 2 g of NaIO 3 and 1 mL of Pb(NO 3 ) 2 (40 mg L -1 ) was added to distilled water (100 mL). The suspension was then irradiated by a 300 W Xe lamp.
After 4 h of photo deposition, the suspension was filtered, washed several times with deionized water, and finally dried in the oven at 60 °C overnight.

Preparation of SiO 2 @HOF core-shell hybrid material:
The amino-functionalized SiO 2 (NH 2 -SiO 2 ) was synthesized according to the literature. 1 In a typical reaction, NTCDA (268.2 mg, 1.00 mmol) and the obtained NH 2 -SiO 2 (200 mg) were dissolved in 10 mL of HAc. Then, the mixture was stirred vigorously and heated at 119 °C for 5 hours. Upon cooling, the resulted solid was washed with DMF and methanol, then lyophilized for 24 hours to obtain anhydride functionalized SiO 2 . In step two, DAT (99.1 mg, 1.00 mmol) was dissolved in 10 mL of DMAc in an ice bath under N 2 for 30 min, then NTCDA (134.1 mg, 0.50 mmol) was added and the mixture was stirred vigorously in an ice bath for another 30 min. Subsequently, the obtained anhydride functionalized SiO 2 and 60 mL of DMAc were added into the mixture, followed by stirring in an ice bath for 5 min. The above was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 170 °C for 6 hours. After cooling to room temperature, the resulted solid was washed with DMF and methanol, then lyophilized for 24 hours to obtain SiO 2 @HOF.

Analytical methods:
The samples were filtered through a microfiltration membrane (13 mm*0.22 μm) before analysis on the high-performance liquid chromatography (HPLC, Agilent 1100). The HPLC equipped with a UV-vis detector at a wavelength of 347 nm and Agilent Extend SB-C18 column (5 µm particle size, 4.6 mm × 250 mm).
The mobile phase was consisted of H 2 O (containing 0.1% formic acid) and acetonitrile at a flow rate of 1 mL min -1 , and the ratio of H 2 O and acetonitrile was 88:12.
A pseudo-first-order kinetic model was used and calculated by Eq. (1) to further compare the photocatalytic efficiencies.
Eq. (1) where C 0 and C t are the initial concentration and the remaining concentration, respectively, of TC (mg L -1 ) at each time point; k is the kinetics rate constant; and t is the reaction time (min). the fitting results of the pseudo-first-order kinetic model in each degradation were supplied in Table S2.

Characterization:
The crystal phase identification of photocatalysts were conducted via an X-ray diffractometer (XRD, Bruker D8 Advanced diffractometer) using Cu-Kα radiation ( = 1.5406 Å) in the 2θ range from 2° to 55°. The microstructures and morphologies were analyzed by microscopy (Hitachi UHR FE-SEM SU8020 scanning electron microscopy and JEM-1200EX transmission electron microscopy).
The molecular structure was analyzed by Fourier transform infrared spectroscopy   hours) without any NTCDA and DAT added. As can be seen in Figure S2b, after the solvothermal process, the main framework of UiO is retained with slight dent and part of which surround ed by small fragments.                        Note: The photo deposition of Au was reduced from H 2 AuCl 4 by acceptance of an electron, and PbO 2 was oxidized from Pb(NO 3 ) 2 by accepting hole. Once U@H2 was irradiated and generated photogenerated electron/hole, the Au 2+ /Pb 2+ would be reduced/oxidized into Au/PbO 2 nanoparticles on the surface. As shown in Figure S22, the Au nanoparticles are successfully deposited on the surface of U@H2, while no PbO 2 nanoparticles could be found. It is indicated that the photogenerated electrons were manlily transfer to the surface of U@H2, i.e., the HOF shell. In addition, the Pb elements appeared at the elemental mapping images were mainly concentrated in the UiO core. It is inferred that the photogenerated hole were manlily maintained in the inner of the U@H2, i.e., the UiO core. There are no obvious coarse particles appear in the images, it probably due to the size limitation of the pore in UiO.